The invention relates to the preparation of trialkylsilyl nitrile (also known as trialkylsilyl cyanide and cyanotrialkylsilane). Trialkylsilyl nitriles, especially trimethylsilyl nitrile, are used to form silylated cyanohydrins from ketones or aldehydes, to conjugatively add cyanide to enones, to form acylnitriles from acylchlorides, to protect ketones, to add to imines to form a-amino nitriles, and to add to oximes to give a-cyano oximes. Trialkylsilyl nitrile will lead to cycloadducts upon reaction with carbodiimides, diimines, isocyanates and isothiocyanates and nitriles. Further, trialkylsilyl nitrile adds to acetylenes, allenes and nitrones and opens oxiranes and oxetanes. It also serves as a source of cyanide ion in various nucleophilic substitutions, both organic and inorganic. Trimethylsilyl nitrile has been described as one of the most versatile silicon reagents for the purposes of organic synthesis. J.K. Rasmussen, S.M. Heilmann, L.R. Krepski, "The Chemistry of Cyanotrimethylsilane," in Advances in Silicon Chemistry (Ed. G.L. Larson), Vol. 1, 66 (1991).
Trialkylsilyl nitrile has been prepared for several years by reaction of the iodosilane (R).sub.3 SiI with silver cyanide according to the reaction ##STR1## Trimethylsilyl nitrile has also been obtained from the corresponding sulfide, bromide and chloride. W.C. Groutas, D. Felker, "Synthetic Application of Cyanotrimethylsilane, Iodo trimethylsilane, Azidotrimethylsilane, and Methylthio-trimethylsilane," Synthesis, Nov., 861 (1980), report obtaining an 80% yield through reaction of silver cyanide with chlorotrimethylsilane. Despite the obvious expense of silver cyanide, it was the reagent of choice for the preparation of trialkylsilyl nitriles for several years. Rasmussen et al. at 68.
Trialkylsilyl nitrile has also been obtained in moderate yields through reaction of chlorotrimethylsilane with potassium mercuricyanide in DMF, reaction of hexamethyldisilazane with hydrogen cyanide, reaction of methoxytriethysilane with pivaloylcyanide in the presence of aluminum chloride, and from cyanide exchange between chlorotriphenysilane and cyanotriethylsilane. Rasmussen et al. at 68-69. The reaction of hexamethyldisilazane with hydrogen cyanide was published by DuPont workers in 1958 in J.Amer.Chem.Soc., 80, 4151-4153 (1958). The DuPont workers disclose a yield of 36.7% trimethylsilyl nitrile and 45% ammonium cyanide. The ammonium cyanide is a nitrile salt and is as such very difficult to handle.
Trimethylsilyl nitrile has been prepared by reacting a diethyl ether solution of trimethylchlorosilane and hydrogen cyanide with a lithium dispersion in petroleum. A modification of this is the subject of U.S. Pat. No. 3,032,575. This has been taken further by first forming lithium cyanide from LiH and hydrogen cyanide, and then preparing trialkylsilyl nitrile from the lithium cyanide.
A moderate yield of 72% trimethylsilyl nitrile has been obtained through reaction of bis(trimethylsilyl)sulfide with dicyanodimethylsilane. M.D. Mizhiritskii, V.0. Reikhsfel'd, Zh. Obshch. Khim., Vol. 55, 1537 (1985). Also, yields of 90-100% have been observed for the synthesis of n-butyldimethylsilyl nitrile, t-butyldimethylsilyl nitrile, and phenyldimethylsilyl nitrile using fluoride ion catalysis. Rasmussen et al. at 71. These reactions involve the following equilibrium: ##STR2##
The high yields were generally obtained by distilling the trimethylchlorosilane as it was formed. Rasmussen et al. at 71. It should be noted that for each mole of trialkylsilyl nitrile formed, a mole of trimethylsilyl chloride is formed as a by-product.
Quantitative yields of trimethylsilyl nitrile have been observed from an equimolar mixture of chlorotrimethylsilane and potassium cyanide. F. Duboudin, P. Cazeau, F. Moulines, O. Laporte, "A New, One-Pot Synthesis of Silylated Cyanohydrins," Synthesis, Mar., 212 (1982). A 71% yield has also been observed for the same reaction, although the authors noted that scrupulous drying of the glassware and solvent were not performed. J.K. Rasmussen, S.M. Heilmann, "A Simple, Safe and Inexpensive preparation of Trimethylsilyl Cyanide," Synthesis, July, 523 (1979). The process, however, generates one mole of potassium chloride as a by-product per mole of trialkylsilyl nitrile. Further, Rasmussen and Heilmann report using anhydrous N-methylpyrrolidone as solvent, which is expensive and difficult to recycle.
The use of hydrogen cyanide is described in B. Uznanski, "An Improved Preparation of Trimethylsilyl Cyanide," Synthesis. Oct., 154 (1978). According to Uznanski, trimethylsilyl chloride is reacted with a 25 mol % excess of hydrogen cyanide and triethylamine in diethyl ether giving a 70% yield of trimethylsilyl nitrile. Uznanski's method produces only moderate yields and generates an equimolar amount of triethylamine hydrochloride as a by-product.
The current state of the art involving cyanide salts is described in U.S. Pat. No. 4,429,145, to M.T. Reetz. The Reetz method employs potassium cyanide. The Reetz method uses anhydrous N-methylpyrrolidone as solvent and 10 mol % potassium iodide as a catalyst. Both chemicals are expensive and difficult to recycle.